This invention relates to automatic sheet feeding devices suitable for use with facsimile systems, electrophotographic copying apparatus, printing presses, packing machines, etc., in which sheets which may include documents, recording sheets, printing sheets, packing sheets, etc., are fed automatically one after another to an operation station which may be a document read-out station, a printing station or a packing station, and more particularly it is concerned with a sheet feeding device for automatically feeding a plurality of sheets placed on a sheet tray in a stack by separating them into individual sheets.
In one type of automatic sheet feeding device known in the art, a portion of an outer peripheral surface of a separation roller protrudes from a sheet supporting surface of a sheet tray, and a separation member is brought into contact with the protruding portion of the outer periphery of the separation roller so as to form a wedge-shaped space defined between the separation roller and the separation member. In this type of automatic sheet feeding device, a stack of sheets are placed on the sheet tray in such a manner that a leading end portion of a lowermost sheet of the stack of sheets is brought into contact with the protruding portion of the outer periphery of the separation roller. Simultaneously as or immediately after the stack of sheets is placed on the sheet tray, the separation roller is rotated in a sheet feeding direction to allow a plurality of sheets to enter the wedge-shaped space from the stack of sheets. As a result, the lowermost one of the plurality of sheets entering the wedge-shaped space continues its forward movement by virtue of a sheet feeding force produced by friction acting between the lowermost sheet and the separation roller while the sheets overlying the lowermost sheet are prevented from continuing their forward movements by friction acting between one of the sheets and the separation roller and friction acting between the sheets themselves. Thus, the lowermost sheet of the plurality of sheets entering the wedge-shaped space is automatically separated from the overlying sheets, so that only one sheet is fed at a time to a position disposed downstream of the separation roller and the separation member.
The sheets placed on the sheet tray may include sheets of different natures and different conditions. Some sheets may tend to curl toward the separation member, and in some sheets, friction acting between them may be higher than in other sheets. When the stack of sheets placed on the sheet tray tends to curl up, the area of contact between each sheet and the separation roller would be lessened and a predetermined sheet feeding force could not be produced by the friction between the sheet and the separation roller, with a result that a failure to feed a sheet might occur. When the friction acting between the sheets is high, other sheets would follow the sheet moved forwardly in a sheet feeding direction by the separation roller, causing a redundant sheet feeding to occur.
The separation member in contact with a surface of the protruding portion of the outer periphery of the separation roller and cooperating therewith to define therebetween the wedge-shaped space as described hereinabove is moved slightly away from the separation roller by a plurality of sheets entering the wedge-shaped space. As the number of sheets fed to the operation station increases, the number of sheets entering the wedge-shaped space also increases. In this case, the separation member might be moved away from the separation roller a distance greater than is necessary, so that it would become impossible to force the plurality of sheets, particularly the lowermost sheet, against the separation roller with a predetermined force, making it impossible to feed one sheet after another to the operation station.
To obviate the aforesaid problem of redundant sheet feeding and no sheet feeding, proposals have hitherto been made to provide an auxiliary feed roller in a position upstream of the separation roller with respect to the sheet feeding direction to aid in moving the sheets in the predetermined direction, or to increase the diameter of the separation roller to increase the area of the peripheral surface of the separation roller, to enable a sufficiently high sheet feeding force to move the lower sheet in the sheet feeding direction to be produced by friction.
These proposals have, however, had the disadvantage that the construction of the mechanism becomes complex or the apparatus becomes too large in size.